The present invention concerns in general terms a method of establishing a radio link in a mobile radio telecommunication network. More precisely, the invention concerns a method of establishing a radio link between an access network controller (RNC) and a base station (B node) of the UMTS network.
FIG. 1 depicts highly schematically the access network in the context of UMTS, known as UTRAN (standing for UMTS Terrestrial Radio Access Network). The access network provides the link between the mobile terminal of the user (UE, standing for User Equipment) and the interconnection network (CN, standing for Core Network). It has access network controllers (RNC, standing for Radio Network Controller) and base stations (B nodes), each RNC controller being able to control a plurality of B nodes. The interface between a B node and the RNC which controls it is referred to as the Iub interface.
The RNC controller is responsible for managing the radio resources and controlling the handover. For this purpose, the RNC controller establishes or releases AAL2 connections to the Iub interface. When a mobile terminal is in a soft-handover situation, it communicates simultaneously with several base stations, these base stations being able to be controlled by the same RNC controller or different controllers. When the mobile terminal moves, it is therefore necessary to create new AAL2 connections and to release others, and this rapidly, so as not to create an interruption for the user.
FIG. 2 depicts the different protocol planes used on the Iub interface, as specified by the 3GPP working group on the UTRAN access network. The Iub interface has recourse to two distinct protocol layers: a transport layer (I) and a layer related to the radio functions (II).
The first protocol plane (RNCP standing for Radio Network Control Plane) concerns the transportation of UMTS control messages. It includes the NBAP (standing for Node B Application Part) application protocol for negotiating radio resources between the B node and the RNC controller in order to support the communications of the users. The messages of this protocol are transmitted over the ATM link via adaptation layers.
The second protocol plane (TNCP standing for Transport Network Control Plane) concerns the transportation of the control messages of the transport layer. It comprises the ALCAP (Access Link Control Application Part) protocol providing independence between the control plane (RNCP) and the user plane. The messages of the ALCAP protocol are transmitted on the ATM layer via adaptation layers.
The third protocol plane (UP standing for User Plane) concerns the user traffic flows. It comprises logic channels (LCH), common or dedicated, and an AAL2 layer providing the adaptation to the ATM layer.
FIG. 3 depicts schematically the procedure for establishing a radio link. The NBAP protocol layer of the RNC initiates the procedure by transmitting a request (NBAP: RL Set-up Request) specifying the number of AAL2 connections to be established on the Iub interface. The base station (B node) responds in return by means of a response message (NBAP: RL Set-up Response) supplying to the controller an ATM connection identifier. The content of the messages RL Set-up Request and RL Set-up Response is described in the document 3GTS25.433 from the 3GPP Special Working Group on the Radio Access Network entitled “UTRAN Iub Interface NBAP Signalling”.
The ALCAP protocol layer of the RNC next generates AAL2 connection requests equal in number to the number required by the NBAP layer. More precisely, each of the requests (Q.AAL2(ALCAP): ERQ) is transmitted by the RNC in accordance with Q.2630.1 signalling, also referred to as Q.AAL2, and gives rise to a corresponding confirmation (Q.AAL2(ALCAP): ECF) on the part of the base station, each confirmation including the identifier for the connection.
The procedure for establishing a radio link between a base station and an RNC controller is therefore relatively cumbersome since it requires the intervention of two protocol levels (NBAP and ALCAP) and the exchange of 2(N+1) messages, where N is the number of AAL2 connections to be established to support the said link. However, as has been seen, this establishment procedure must be rapid.